Galvanized steel is used in a wide range of fields such as automobiles, household electrical appliances, building materials, and the like, but from the object of ensuring a long term rust prevention effect, generally plating of a high deposition weight is effective. This is because galvanization itself slows the corrosion rate in a steel material, in addition, zinc, whose corrosion potential is low, has a sacrificial protection in regards to the steel material at places where the iron metal is exposed, and the anti-corrosion effect due to these is obtained by the consumption of zinc, so the greater the amount of zinc per unit area, the longer the effect can be maintained. Recently, it has been learned that the corrosion products of zinc themselves also have the effect of suppressing the corrosion of the plated layer and iron metal, but this effect is also related to the absolute amount of the zinc, so again a greater amount of zinc is desired. On the other hand, if the amount of zinc deposition becomes great, the workability, weldability, and other required characteristics of the steel material tend to degrade. If possible, use of a lower amount of deposition to obtain a high corrosion resistance is desired.
To impart a sufficient corrosion resistance by a lower amount of deposition of plating, many attempts have been made to raise the corrosion resistance of galvanization by adding alloy elements. In actuality, Zn—Ni alloy platings, Zn—Fe alloy platings, and the like are being widely used mainly for automobile steel sheets, while Zn—Al alloy platings are being widely used mainly for building materials. In particular, in Zn—Al alloy platings, plated layers given Mg or Si to further improve corrosion resistance have been developed.
However, if using the addition of alloy components to improve the corrosion resistance of the plated layer, depending on the combination of the type of the alloy components and corrosion environment, the plated layer is sometimes ends up becoming passivated. As described above, in corrosion resistance in a galvanized steel material, both the small corrosion rate of the plated layer itself and the function of sacrificial protection of the sacrificial dissolution of the plated layer protecting the iron metal when the iron metal is exposed are important. If the plated layer is passivated, while the corrosion rate of the plated layer itself becomes extremely small, the function of the sacrificial protection ends up being lost or even conversely the phenomenon arises of the corrosion of the iron metal ending up being promoted if the iron metal is exposed.
Not much has been disclosed in patent documents up until now about the method of obtaining both the contradictory functions of higher corrosion resistance of the plated layer itself and securing sacrificial protection when the iron metal is exposed.
For example, Japanese Patent Publication (A) No. 6-248424 discloses the idea relating to Zn—Mg alloy plating of giving a gradient to the distribution of concentration of the alloy components in the plated layer in the thickness direction to secure high corrosion resistance at the surface layer and sacrificial protection near the iron metal. This idea would be excellent if it could be attained by an inexpensive method, but to give a gradient to the components in the thickness direction of the plated layer, vapor deposition plating or other relatively expensive, poor productivity methods have to be employed. Use of the superior productivity hot dip plating to give a gradient to the components in the thickness direction of the alloy plating is very difficult. Gradient components plating by alloying with the iron metal such as with hot dip galvannealing is possible, but what can be controlled by alloying with iron metal is basically the ratio of the Fe element and the other plating components, so the obtainable corrosion resistance remains in the range of the plating components mostly containing Fe, and a sufficient high corrosion resistance cannot be hoped for.
Further, Japanese Patent Publication (A) No. 6-346254 discloses a method of using a multi-layered plating structure comprised of a base metal plated with Zn by vapor deposition and further plated by Cr by vapor deposition so as to reduce the corrosion rate of Zn and maintain the sacrificial protection for a long time. This is also a method of vapor deposition plating.
Japanese Patent Publication (A) No. 2001-234361 discloses, while not galvanization, a method of Ni—Au multi-layered plating suppressing corrosion of the underlying Ni layer by high corrosion resistance Au by providing the underlying Ni layer with a precious Ni layer at the bottom and a base Ni layer at the top and using the sacrificial protection function of the top base Ni layer to improve the overall corrosion resistance. Applying this idea to hot dip galvannealing would again require a multi-layered structure, so a large rise in production costs cannot be avoided.
The technologies disclosed in said Japanese Patent Publication (A) No. 6-346254 and Japanese Patent Publication (A) No. 2001-234361 are both ideas of separation of the functions of corrosion resistance and sacrificial protection by multi-layered platings having layered structures. The technology disclosed in Japanese Patent Publication (A) No. 6-248424 can be said to be a similar idea in the point of separation of functions between the top layer part and the bottom layer part. That is, up to now, the only idea has been the separation of the two contradictory functions among several layers.
Furthermore, one of the problems that occurs when improving the corrosion resistance of the plated layer by the addition of alloy components is the defect that the alloy plating is generally poor in workability. In particular, the greater the number of types of elements added, such as a third element or fourth elements, the easier the formation of poor ductility intermetallic compounds or supersaturated solutes etc. the poorer the workability tends to become.
Further, component elements added for improving the corrosion resistance also form intermetallic compounds of different compositions or cause precipitation of intermetallic compounds of different compositions from the matrix in the matrix of the solutes. In that state, a pair of intermetallic compounds with the different composition or a pair of the matrix and an intermetallic compounds forms electrochemical coupling cells of corrosion whereby conversely corrosion sometimes ends up being promoted.
To deal with the negative effects caused by the formation of these intermetallic compounds etc., if it were possible to make the metal structure of the alloy plating amorphous, no intermetallic compounds would be formed, so the negative effects could be eliminated and it would be possible to realize an alloy plated steel sheet with high corrosion resistance and excellent workability.
However, usually, to produce amorphous metal, there is a large problem that the cooling rate required for the formation of an amorphous structure is extremely large.
For example, Japanese Patent Publication (A) No. 2005-126795 discloses a method of forming an amorphous covering comprising flame spraying a metal powder having Mg and Zn as chemical components on a base material so give a cooling rate of 100,000° C./sec or more.
In this way, in a usual amorphous alloy, when solidifying from a molten state, a large cooling rate is necessary. In the conventional hot dip plating the cooling rate in the temperature range when solidifying from the molten state of plating is said to be 100° C./sec or less, formation of an amorphous structure in the plating state is not possible. With the method of production of flame spraying, it is difficult to inexpensively mass produce a uniform material.
Japanese Patent Publication (A) No. 2005-60805 discloses a film-like alloy member containing an amorphous phase formed by making amorphous alloy particles, comprised of an alloy system of an Fe, Co, and Ni-based alloy, but able to include Zn up to 20 at % as a selectively added element and having an amorphous phase in terms of volume percent of 50% or more, strike a substrate at a high speed. This method is also a method of producing amorphous alloy particles and formation of a film on a substrate having inefficiencies similar to flame spraying and cannot be said to be a method suited for inexpensive mass production.
Regarding the issue of the cooling rate, in recent years alloy compositions able to be made amorphous even with a small cooling rate have been discovered and energetically researched. Such an alloy that can be made amorphous even if solidified at a small cooling rate can be made amorphous even if an alloy of a relatively large size since the cooling rate is small, so is called “bulk amorphous”. If the cooling rate required for formation of an amorphous structure becomes sufficiently small and an amorphous alloy could be formed by a cooling rate able to be realized by a conventional hot dip plating process or less, there would be the possibility of amorphous plating by hot dip plating.
However, up until now, the only reported examples of alloy systems forming bulk amorphous structures have been based on Zr, Mg, Fe, Pd, Co, Ca, and other limited elements. There have been no examples of Zn-based alloys.
There have been some examples of Zn as an element selectively added to a bulk amorphous structure based on other elements. For example, as in Japanese Patent Publication (A) No. 2006-2252, an Mg-based bulk amorphous alloy containing Zn as a selective element in an amount of up to 30 at % and, as in Japanese Patent Publication (A) No. 2004-149914, a Zr/Hf-based bulk amorphous alloy containing Zn as a selective element in an amount of 5 to 15 at % have been disclosed.
However, there is no example of a bulk amorphous alloy containing Zn, required for corrosion resistance of a hot dip plated steel sheet, as its main component. Further, there is no example of a combination of elements considering the performance and productivity of hot dip plated steel sheet. Consequently, there are no reported examples or examples of patents of amorphous Zn-based hot dip plated steel sheet.